1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) communication system. In particular, the present invention relates to an apparatus and method for allocating frequency resources by taking mobility of a user equipment (UE) into account to support High Speed Downlink Packet Access (HSDPA) service.
2. Description of the Related Art
With the recent development of various content for mobile communication service, there is an increasing demand for multimedia service such as video, audio, text and digital broadcasting, as well as voice service. To meet the demand, research is being conducted on Orthogonal Frequency Division Multiplexing (OFDM), which is a conventional radio access technology for efficiently transmitting high-speed data. OFDM is a typical technology for the 4th, or later, generation mobile communication system, and research into and standardization on ultra-high speed packet transmission technologies are being actively performed worldwide.
OFDM technology provides high frequency efficiency with the use of multiple orthogonal subcarriers, and ensures high-speed data processing with the use of an inverse fast Fourier transform (IFFT) block and a fast Fourier transform (FFT) block. In the OFDM scheme, transmission is performed per OFDM symbol, and current transmission symbols may occasionally be affected by previous transmission symbols in a multipath environment. To prevent interference between OFDM symbols, the OFDM scheme uses a cyclic prefix (CP) to provide robustness against symbol interference and fading in the multipath channel environment. Advantageously, the OFDM scheme is simply extensible to multiuser and multiantenna systems.
In this context, 3rd Generation Partnership Project (3GPP) Rel6 (Release 6) provide use of the OFDM scheme for downlink transmission of high-speed packet data. That is, the OFDM scheme is used for supporting High Speed Downlink Packet Access (HSDPA) service. In this case, to transmit downlink OFDM signals, a base station includes a function of converting frequency-domain signals into time-domain signals using IFFT, a function of modulating data carried on the OFDM signals, and a function of processing a radio frequency (RF) related to transmission of the OFDM signals.
FIG. 1 illustrates a conventional structure of a base station transmitter proposed in 3GPP Rel6.
Referring to FIG. 1, a base station supporting HSDPA provides a high speed-downlink shared channel (HS-DSCH) for data transmission, and a high speed-shared control channel (HS-SCCH) for carrying control information needed for demodulation of the HS-DSCH. For example, the HS-SCCH carries channelization code set (CCS) information indicating the number of codes, through which the HS-DSCH has been transmitted. In the current standard, a maximum of 15 channelization codes can be used for transmission of the HS-DSCH.
HS-DSCH data for HSDPA and HS-SCCH information including control information and pilot information are mapped to Quadrature Amplitude Modulation (QAM) signals in QAM mappers 100 and 102, respectively. A physical channel (PHY_CH) mapping and multiplexing block 104 maps the QAM signals to subcarriers for data transmission and subcarriers for control information transmission, and then multiplexes the mapped subcarriers. An IFFT block 106 performs IFFT, i.e., OFDM modulation, on the N multiplexed subcarrier signals, and outputs an OFDM symbol comprised of N OFDM samples.
A guard interval inserter 108 inserts a prefix or a suffix in the OFDM symbol comprised of N OFDM samples. For example, the guard interval inserter 108 copies the last G OFDM samples (copied sample data) among the OFDM samples and inserts the copied sample data in the front of the OFDM symbol. Herein, the OFDM symbol, in the front of which a prefix or a suffix is inserted, will be referred to as an “OFDM transmission symbol.” An OFDM transmission symbol output from the guard interval inserter 108 is separated into an in-phase (I) channel and a quadrature-phase (Q) channel, and converted into an analog OFDM signal by a digital-to-analog converter (DAC) 110. The OFDM signal is converted into a radio frequency (RF) signal by a RF unit 112 and then transmitted through an antenna 114.
Reception performance of the OFDM signal depends considerably on the mobility of a UE receiving the OFDM signal. This is because a radio channel changes characteristics according to movement and location of the UE.
If a moving velocity of the UE becomes higher than or equal to a threshold velocity increasing a Doppler effect, the channel quality of the UE is subject to frequent variation, so that the base station cannot normally transmit downlink packet data. That is, the conventional system allocates radio resources for data transmission without considering the mobility of UEs, causing deterioration of the entire system performance.